Circuit breaker with thermal trip device of high short-circuit withstandability



H. E. S. OWEN April l1, 1967 INVENTOR. HENRY E. S. OWEN FleQ/x Flai-5A BY @Mwy Filed July 1, 1964 CIRCUIT BREAKER WITH THERMAL TRIP DEVICE OF HIGH SHORT-CIRCUIT WITHSTANDABILTY FIGn A TTOR/VEY ,through for a predetermined time.

United States Patent() CIRCUIT BREAKER WITH THERMAL TRIP DEVICE F HIGH SHORT-CIRCUIT WITH- STANDABILITY Henry E. S. Owen, Unionville, Conn., assignor to General Electric Company, a corporation of New York Filed July 1, 1964, Ser. No. 379,648 2 Claims. (Cl. 200--116) My invention relates to electric circuit breakers, and particularly to electric circuit breakers of the type incorporating a thermallyl responsive element, such as a bimetallic strip, to initiate automatic opening of the circuit breaker upon the occurrence of predetermined current conditions.

In electric circuit breakers incorporating current responsive elements of the bimetallic strip type, the bimetallic strip is ordinarily heated by current passing therethrough, or through an adjacent heater, and is ar- .ranged to be warped or deflected when heated to cause load current conditions which persist for given lengths of time, but also to withstand and to interrupt currents of short-circuit magnitude. As previously mentioned, the bimetallic strip generates heat in response to current passing therethrough. On the occurrence of short-circuit currents, therefore, a large amount of heat is generated very quickly in such a bimetallic strip, and at times the bimetallic strip is destroyed by the high temperature, much as a fuse link is destroyed. This is especially true yin applications where the available short-circuit current f is extremely high, and in circuit breakers which have relaf'tively high normal current-carrying ratings.

It is an object of the present invention to provide an electric circuit breaker of the thermal actuated type utilizing a bimetallic strip which is able to successfully withstand high short-circuited currents without being de- ,stroyed Itis another object of the invention to provide an electric circuit breaker of the type described which includes means for compensating for the effect of relatively high ambient Itemperatures.

In accordance with the invention in one form, an electric circuit breaker is provided including a pair oi sepvarable contacts and operating mechanism for normally retaining the :contacts in closed circuit condition.

A thermally-responsive trip device is also provided including an .elongated bimetallic strip which is so arranged as to cause release of the operating mechanism when the temperature ofthe bimetallic strip is raised a given amount such as by the passage of a predetermined current there- In addition, the bimetallic strip is shunted or paralleled by a conductive ,member which has a relatively high electrical resistivity,

but relatively low total resistance. The `relatively low total electrical resistance is achieved by utilizing a conductor having relatively large cross-section. By this general arrangement, a shunt is provided which has a relajtively large thermal capacity and so is not readily destroyed by high currents, and one which has substantial mechanical strength and therefore is not readily warped by high currents, or by magnetic force effects.

In accordance with another aspect of the invention, the shunting material is selected from the class of mate- Patented Apr. 1l, 1967 rials having a relatively low thermal co-efticient of resistivity compared to that of the bimetallic strip. This characteristic, which is also known as temperature coefficient of resistance is a measure of the change of resistance accompanying a change in temperature. This has the effect of affording an ambient temperature cornpensation action, since as the ambient' temperature increases, the lresistance of the bimetallic strip increases relatively more rapidly than that of the shunt. A smaller portion of the total current is therefore carried by the bimetallic strip at higher ambient temperatures, and the amount of heat generated in the bimeta-l thereby is correspondingly reduced. The net effect, in other words, is that the bimetallic strip acts as though it were less sensitive at high ambient temperatures. By this means, the circuit breaker is enabled to carry its proper rated current in high ambient temperature condition although the ambient temperature is contributing more heat to the .current responsive bimetallic strip, since a corresponding portion of the current is being by-passed around the bimetallic strip, thereby compensating for this heating action.

In accordance with a further aspect of the, invention, a magnetic tripping device is also included in the combination and the series combination of the bimetallic strip and the magnetic current responsive means is by-passed by the shunting means, so `that both of these current responsive elements are protected from the eects of high short-circuit currents, and both benet by the ambient compensation action previously described.

The invention will be more fully understood from the following detailed description, and its scope will be pointed out in the appended claims.

In the drawing,

FIGURE 1 is a side elevation view of an electric circuit vbreaker incorporating the invention, a portion of the side wall being broken away to show the parts;

FIGURE 2 is a perspective view of the thermal and magnetic tripping elements of the circuit breaker of FIGURE 1;

FIGURE 2A is a schematic drawing illustrating the electrical connections of the components illustrated in FIGURE 2; Y

FIGURE 3 is a side elevation view of a thermal and magnetic tripping means in accordance with another `embodiment of the invention;

A13 respectively. The terminal 13 has connected thereto a terminal strap 14 supporting a stationary contact 15. A movable contact 16 is carried by a contact arm 17 which in turn is pivotally supported at the pivot pin 1-8 on a contact operator- 19 carried by a shaft 20 pivotally supported in the base 10. The movable contact arm 17 is -moved between open and closed circuit positions by suitable operating mechanism, not shown, which may be of conventional construction. Such an operating mecha- `nism is shown, for example, in Patent No. 3,005,066

Powell, issued Oct. 17, 1961, and assigned to the same y assignee as the present invention. The operating mechanism includes a trip bar 22 which, when rotated clockwise as viewed in FIGURE 1, causes automatic opening movement of the movable contact 1'6.

For the purpose of causing releasing movement of the trip bar 22 upon the occurrence of predetermined current conditions, in accordance With the invention, thermal and magnetic tripping means are provided. The thermal tripping means includes an elongated bimetall-ic strip 24 having a bent-over end portion 25 (see FIG. 2) iixedly anchored to the base by suitable means such as by a screw, not shown. The end 25 of the bimetallic strip 24 has connected thereto one end 26 of a magnetic trip coil 27. The other end 28 of the coil 27 is connected to a conductive strap 29 fixed to the base 10` by means of a screw 30. The strap 29 is connected by a flexible conductor 31 to the movable contact arm 17.

T-he input end of the bimetallic strip 24 is connected by means of an insulated flexible conductor 32 to a conductive strap 33 supporting the input terminal 12.

Thus the current path through the circuit breaker may be traced from the input terminal 12, to the conductive plate 33, to the iiexible conductor 32, to the input end of the bimetallic strip 24, to the input end 26 of the coil 27, to the conductive strap 29, through the flexible cond-uctor 31 to the contact arm 17, and thence through the Vcontacts 16 and 15 and the conductive terminal strap 14 to the output terminal 1'3.

The coil 27 serves to energize a magnetic assembly comprising a core or eld piece 35 and pole pieces 35A at each end thereof. The magnetic field piece 35 and pole pieces 35A serve to attract an armature 37 which is pivotlally supported by lugs 38 in notches 39 in the base 10. The upper end 40` of the armature 37 is disposed and arranged to engage the trip bar 22 when the armature is attracted to the eld piece 35, to rotate the trip bar 22 to cause tripping. f

The bimetallic strip 24- likewise carries an actuating or calibrating screw 24A which engages the trip bar 2-2 when the bimetallic strip deflects, and rotates the trip bar 22 clockwise to cause tripping. In accordance with the invention, .a rigid metallic shunt 42 is also provided, which is connected at one end to the bimetallic strip 24 by means of the flexible conductor 43, and at the other end to the strap 29 which also supports and connects the end 28 of the coil 27. The interconnection of the components is therefore as illustrated schematically in FIG- URE 2A, from which it will be seen that the series combination of the bimetallic strip 24 and the coil 27 is paralleled or by-passed by the shunt 42, and that this series parallel combination is in series with the current owing through the contacts 15, 16.

Short-circuit wthstundability In accordance with the invention, the shunt 42 is preferably constructed of a metallic material having a relatively high electrical resistivity. By ut-ilizingv a material having a relatively high electrical resistivity, a piece of large cross-secton may be used to provide the resistance desired of the total piece. As a result, the mass of the shunt is relatively large. Because the mass of the shunt is relatively large, it has a correspondingly relatively large thermal capacity. Therefore, during short circuit conditions, it is capable of absorbing a large amount of heat without melting or deforming. Thus, for example, .a ,circuit breaker rated at 40 amperes normal currentcarrying capacity, constructed in accordance With the invention, successfully withstands let-through current which yoccurs when a short-circuit current having an availabie value of 10,000 amperes is interrupted. In this instance, a shunt constructed of nickel-chromium alloy material 'is used, having an electrical resistivity of 650 ohms per circular mil foot. The resistivity of copper, by comparison, is about 10 ohms per circular mil foot.

In addition to the property of relatively high electrical resistivity, I preferably use as a shunt material a material having a relatively low thermal co-efiicient of resistivity. The use of a material having a relatively low thermal co-efticient of resistivity has two important benefits: (l) it provides an ambient ,temperature compensation action,

and (2) it provides a beneiicial shifting of the split lof currents through the shunt and the bimetallic strip during short-circuit conditions.

Ambient temperature compensation action In accordance with this aspect of the invention, a material is used for the shunt 42 which has a relatively low thermal co-efiicient of resistivity, as compared to the material used for the bimetallic strip 24. Thus, for eX- ample in a typical application, I may utilize a bimetallic strip which undergoes a change of resistance of 112.8% as the temperature goes from zero to 700 degrees Fahrenheit, in combination with a shunt which undergoes a change of resistance of only 1.4% for the same temperature change. As the ambient temperautre increases, therefore, the total resistance of the bimetallic strip 24 will increase more appreciably than the total resistance of the shunt 42. Thus at higher ambient temperatures, a greater percentage of the current will be by-passed through the shunt 42. Thus the total current at which the breaker trips will be more nearly thesame whether the ambient temperature is relatively high or relatively low. In the case where the ambient temperature vis relatively low, the current passing through the bimetallic strip must do substantially all of the heating of the strip, and therefore a larger portion of the total current is needed. At relatively high ambient temperatures, however, the ambient temperature itself will tend to raise the temperature of the bimetallic strip, and therefore' only a smaller portion of the total current is needed to bring the bimetallic strip to a trip condition.`

Shifting of current division during short-circuit conditions In accordance -With the invention, the shunt 42l preferably constructed so that the division of current at normal current ratings of about 40 amperes is 50% through the bimetallic strip and 50% through the shunt 42. On the occurrence of short-circuit current conditions, it is desired that the bimetallic stripv should be protected from excess heating caused by such current flowing even during a short time required for the contacts to open and interrupt the current. Since the shunt 42 has a relatively greater thermal mass, and also since slight distortion yof the shunt due to high heating is not of any particular concern as compared to possible distortion of the bimetallic strip, the shunt can tolerate a relatively greater proportion of current during the short-circuit conditions than the bimetallic strip. By using materials such that the thermal `co-eilici'ent of resistivity of the shunt is relatively low compared to that of the bimetallic strip, a condition results such that as the temperature of the two pieces increases due to short-circuit currents, the total resistance of the bimetallic strip increases relatively faster than the total resistance of the shunt. The result is that a greater proportion of the total current passes through the shunt and a lesser portion through the bimetallic strip.

In FIGURE 3, there is shown another embodiment of the invention. In this form of the invention, the shunt 142 is used as a support for the bimetallic strip 124, both of these parts being anchored to the base by the screw 110A. The other end of the shunt 142 and the other end of the bimetallic strip 124, are both connected to the terminal strap 133. Thus the bimetallic strip.12`4 is directly paralleled by the shunt 142, as indicated schematically in FIGURE 3A. The magnetic trip coil 127 is then in series with this combination. This arrangement has the advantage that the shunt 142 serves not only as a shunt, .but also as an indirect heater to contribute heat to the bimetallic strip 124. This is extremely useful in relatively low-rated lbreakers such as 15 amperes or below, since it permits the use of a heavier bimetallic strip which therefore has adequate strength to perform the function of tripping. It will be appreciated that the shunt 142 in this form -of the invention is also preferably constructed of a material having a relatively low thermal co-eiiieient of resistivity, and therefore provides the additional beneits of ambient compensation and current shifting during short-circuit conditions as described above in connection with the form of FIGURE 2.

A third arrangement of the components illustrated is indicated at FIGURE 4. In this form of the invention, the bimetallic strip 224 is paralleled by the series combination of the shunt 242 and the magnetic coil 227. This arrangement includes the resistance and impedance of the coil 227 -in the shunting path and has the advantage that the total resistance of the shunt 242 may 'be relatively lower than otherwise, since the resistance of 4the coil is in series with it, and therefore a shunt having greater thermal mass may be used.

While the invention has been disclosed Iin only certain particular embodiments, it will be readily apparent that many modiiications thereof may readily be made, and I therefore intend by the appended claims to cover all such modifications as fall Within the true spirit and scope of the invention.

What I claim as new and desi-re to secure by Letters Patent of the United States is:

1. An electric circuit breaker comprising:

(a) an insulating casing,

(b) at least one pair of relatively separable contacts supported in said insulating casing,

(c) operating mechanism for operating said separable contacts, said operating mechanism including trip means movable to cause automatic opening of sa-id contacts, y

(d) an elongated bimetallic strip supported in said insulating casing, and having a thermal coeihcient of resistivity of not less that .01% per degree Fahrenheit,

(e) 'means for heating said bimetallic strip in response to current passing -through said contacts,

(t) means connecting said bimetallic strip to said trip means to cause actuation of said trip means and automatic opening of said` conta-cts upon predetermined heat-ing of said bimetallic strip;

(1g) shunting means electrically in parallel with Vsaid heating means for said bimetallic strip, said shunt being constructed of a metallic material having a thermal co-eiicient of resistivity of not more than .002% per degree Fahrenheit.

2. An electric circuit breaker comprising:

(a) an'insulating casing,

(b) at least one pair of relatively separable contacts mounted in said insulating casing, Y

(c) operating mechanism in said insulating casing for operating said relative-ly `separable contacts between open and closed circuit positions, said operating mechanism including a trip member movable to cause automatic opening of said contacts,

. (d) an elongated bimetallic strip mounted in said insulating casing and including a portion disposed and arranged to engage said trip member upon heating of `said bimetallic strip to a predetermined extent to cause actuation of said trip member and automatic opening of said contacts upon predetermined heating of said bimetallic strip,

(e) a `shunt connected electrically in parallel `with said bimetallic strip, said shunt having an electrical resistivity of not less than 500 ohms per circular mill foot,

(f) -said bimetallic strip having a thermal co-efiicient of resistivity of not less than .01% per degree Fahrenheit, and

(g) said shunt having a thermal co-eiiicient of resistivity of not more than .002% per degree centigrade.

References Cited by the Examiner UNITED STATES PATENTS 1,704,379 3/1929 Aichele 200-116 1,732,295 10/1929 Aichele 200-116 1,905,501 4/1933 Randall 200-122 1,966,051 7/1935 Sperry et `al 200-116 2,050,284 8/1936 Dorfman 200e-122 X 2,074,584 3/ 1937 Graves 200-88 2,300,900 11/ 1942 Armstrong 200-88 2,354,162 7/1944 Weber 200-88 2,439,069 4/ 1948 Anderson 200-116 2,454,106 11/1948 Von Hoorn 200-116 2,629,796 2/1953 Kern 200-116 3,201,543 S/1965 Leonard 200-88 FOREIGN PATENTS 347,073 4/11931 Great Britain.

BERNARD A. GILHEANY, Primary Examiner.

H. GILSON, Examiner. 

1. AN ELECTRIC CIRCUIT BREAKER COMPRISING: (A) AN INSULATING CASING, (B) AT LEAST ONE PAIR OF RELATIVELY SEPARABLE CONTACTS SUPPORTED IN SAID INSULATING CASING, (C) OPERATING MECHANISM FOR OPERATING SAID SEPARABLE CONTACTS, SAID OPERATING MECHANISM INCLUDING TRIP MEANS MOVABLE TO CAUSE AUTOMATIC OPENING OF SAID CONTACTS, (D) AN ELONGATED BIMETALLIC STRIP SUPPORTED IN SAID INSULATING CASING, AND HAVING A THERMAL COEFFICIENT OF RESISTIVITY OF NOT LESS THAT .01% PER DEGREE FAHRENHEIT, (E) MEANS FOR HEATING SAID BIMETALLIC STRIP IN RESPONSE TO CURRENT PASSING THROUGH SAID CONTACTS, (F) MEANS CONNECTING SAID BIMETALLIC STRIP TO SAID TRIP MEANS TO CAUSE ACTUATION OF SAID TRIP MEANS AND AUTOMATIC OPENING OF SAID CONTACTS UPON PREDETERMINED HEATING OF SAID BIMETALLIC STRIP; (G) SHUNTING MEANS ELECTRICALLY IN PARALLEL WITH SAID HEATING MEANS FOR SAID BIMETALLIC STRIP, SAID SHUNT BEING CONSTRUCTED OF A METALLIC MATERIAL HAVING A THERMAL CO-EFICIENT OF RESISTIVITY OF NOT MORE THAN .002% PER DEGREE FAHRENHEIT. 